Mark Rowe reports on the increasingly ambitious geo-engineering projects being explored by scientists

Mark Rowe就科学家正在开发并且越发受到关注的地球工程项目进行报道

ASuch is our dependence on fossil fuels, and such is the volume of carbon dioxide already released into the atmosphere, that many experts agree that significant global warming is now inevitable. They believe that the best we can do is keep it at a reasonable level, and at present the only serious option for doing this is cutting back on our carbon emissions. But while a few countries are making major strides in this regard, the majority are having great difficulty even stemming the rate of increase, let alone reversing it. Consequently, an increasing number of scientists are beginning to explore the alternative of geo-engineering - a term which generally refers to the intentional large-scale manipulation of the environment. According to its proponents, geo-engineering is the equivalent of a backup generator: if Plan A - reducing our dependency on fossil fuels – fails, we require a Plan B, employing grand schemes to slow down or reverse the process of global warming.

A 我们对于化石燃料的依赖，以及我们已经释放到大气中的二氧化碳量，使得很多专家一致认为严重的全球变暖已经不可避免。他们相信我们最多能做的是将其维持在合理的水平，而目前唯一可以考虑的做法是降低碳排放量。但尽管一些国家正在这方面做出巨大进步，大多数国家即使是控制增长速度也依然非常困难，更不要说扭转这一趋势。因此，越来越多的科学家开始探索实施地球工程——这个术语通常指国际性的大规模环境管理。根据其支持者，地球工程相当于一台备用发电机：如果A计划(减少我们对化石燃料的依赖)失败，我们需要B计划，即开展宏大的计划来控制或扭转全球变暖的进程。

B 地球工程已经被证明有效，至少在较小的地方范围内。几十年来，莫斯科的劳动节阅兵在清澈的蓝天下进行，利用飞机载着干冰、碘化银以及水泥粉来驱散云层。很多方案如今期望用相反的方式进行，以减少到达地球的太阳光。其中亚利桑那大学的Roger Angel教授提出的观点最受人关注。他的方案将使用16兆之多的微型航天器，其中每个重约克，从而在地球上方150万公里的轨道上形成一个透明的、折射太阳光的遮光板。Angel说，这可以将到达地球的光照量减少2％。

BGeo-engineering has been shown to work, at least on a small localised scale. For decades, May Day parades in Moscow have taken place under clear blue skies, aircraft having deposited dry ice, silver iodide and cement powder to disperse clouds. Many of the schemes now suggested look to do the opposite, and reduce the amount of sunlight reaching the planet. The most eye-catching idea of all is suggested by Professor Roger Angel of the University of Arizona. His scheme would employ up to 16 trillion minute spacecraft, each weighing about one gram, to form a transparent, sunlight-refracting sunshade in an orbit 1.5 million km above the Earth. This could, argues Angel, reduce the amount of light reaching the Earth by two per cent.

C 大多数目前已完成的地球工程(包括在沙漠中种植树木以及将铁沉淀入海中来促进藻类生长)希望带来地球整体的降温。但其中一些特别关注阻止两极的融化，尤其是北极。理由是如果你填满高纬度冰川以及冷冻水，更多光将被反射回太空，从而减缓海洋和大气的变暖。

D 很多科学家提出向北极上方平流层释放喷雾剂的想法。这包括使用硫或氢化硫喷雾，由此二氧化硫将形成云层，这将转而导致全球变暗。这一想法模仿了历史上的火山爆发，例如1991年菲律宾皮纳图博火山的爆发，这次火山爆发使短时间内全球气温下降了0．5摄氏度。科学家同样细致探讨了是否可能用强化的高压电缆保护格陵兰的冰层，以防止冰山进入大海。同时在俄罗斯北极地区，地球工程方案包括种植数百万棵桦树。而这一地区本地生长的常青松树遮挡雪并吸收辐射，桦树将在冬天落叶，从而使辐射能够被雪反射。一些气候科学家说，改变俄罗斯河流方向来增加流到结冰区域的冷水同样可以减缓地球变暖。

CThe majority of geo-engineering projects so far carried out - which include planting forests in deserts and depositing iron in the ocean to stimulate the growth of algae - have focused on achieving a general cooling of the Earth. But some look specifically at reversing the melting at the poles, particularly the Arctic. The reasoning is that if you replenish the ice sheets and frozen waters of the high latitudes, more light will be reflected back into space, so reducing the warming of the oceans and atmosphere.

E 但这样的计划会被实施吗?一般而言，参与研究的科学家对地球工程最为谨慎。Angel说他的计划“无法取代开发可再生能源这一唯一的永久解决方案。”美国太平洋西北国家实验室的Phil Rasch博士对地球工程的作用同样持谨慎态度：“我认为我们所有人都同意如果有一天停止地球工程，地球将很快回到实施地球工程前的状态，或许在十到二十年间。这肯定是值得人们担忧的。”

F 美国国家大气研究中心已经表示将硫注入大气的提议可能影响热带地区和南大洋的降雨模式。“注入平流层喷雾或制造云层的地球计划将会使地球冷却，并将增加海上浮冰的范围，”Rasch说。“但所有模式都意味着对降水的分布产生一些影响。”

DThe concept of releasing aerosol sprays into the stratosphere above the Arctic has been proposed by several scientists. This would involve using sulphur or hydrogen sulphide aerosols so that sulphur dioxide would form clouds, which would, in turn, lead to a global dimming. The idea is modelled on historic volcanic explosions, such as that of Mount Pinatubo in the Philippines in 1991, which led to a short-term cooling of global temperatures by 0.5 °C. Scientists have also scrutinised whether it's possible to preserve the ice sheets of Greenland with reinforced high-tension cables, preventing icebergs from moving into the sea. Meanwhile in the Russian Arctic, geo-engineering plans include the planting of millions of birch trees. Whereas the region's native evergreen pines shade the snow and absorb radiation, birches would shed their leaves in winter, thus enabling radiation to be reflected by the snow. Re-routing Russian rivers to increase cold water flow to ice-forming areas could also be used to slow down warming, say some climate scientists.

G 布里斯托大学地球物理科学学院的Dan Lunt博士说：“地球工程项目进一步的风险是可能会‘注入过多’”，他已经研究了遮光板和气雾剂方案对气候的影响。“你可能会将全球气温带回到工业化前的水平，但风险在于极地将依然比其应有温度更高而热带地区的温度将低于工业化之前。”为避免这样的情况，Lunt说Angd的项目需要以一半力量来实施；这一切都符合他的观点：最好的选择是避免整体性的地球工程。

H 地球工程受到很多科学界人士支持的主要原因是多数研究者并不认为政治家会认可并随之带来必要的碳排放减少。即使领先的环境组织也关注研究地球工程潜力的价值。根据世界野生动物基金会国际北极项目的气候变化顾问Martin Sommerkom博士的观点，“人类引起的气候变化已经将人类带到一种必须彻底讨论这一话题及其可能性的处境。”

EBut will such schemes ever be implemented? Generally speaking, those who are most cautious about geo-engineering are the scientists involved in the research. Angel says that his plan is 'no substitute for developing renewable energy: the only permanent solution'. And Dr Phil Rasch of the US-based Pacific Northwest National Laboratory is equally guarded about the role of geo-engineering: 'I think all of us agree that if we were to end geo-engineering on a given day, then the planet would return to its pre-engineered condition very rapidly, and probably within ten to twenty years. That's certainly something to worry about.'

FThe US National Center for Atmospheric Research has already suggested that the proposal to inject sulphur into the atmosphere might affect rainfall patterns across the tropics and the Southern Ocean. 'Geo-engineering plans to inject stratospheric aerosols or to seed clouds would act to cool the planet, and act to increase the extent of sea ice,' says Rasch. 'But all the models suggest some impact on the distribution of precipitation.'

G'A further risk with geo-engineering projects is that you can "overshoot", 'says Dr Dan Lunt, from the University of Bristol's School of Geophysical Sciences, who has studied the likely impacts of the sunshade and aerosol schemes on the climate. 'You may bring global temperatures back to pre-industrial levels, but the risk is that the poles will still be warmer than they should be and the tropics will be cooler than before industrialisation.' To avoid such a scenario, Lunt says Angel's project would have to operate at half strength; all of which reinforces his view that the best option is to avoid the need for geo-engineering altogether.

HThe main reason why geo-engineering is supported by many in the scientific community is that most researchers have little faith in the ability of politicians to agree - and then bring in - the necessary carbon cuts. Even leading conservation organisations see the value of investigating the potential of geo-engineering. According to Dr Martin Sommerkorn, climate change advisor for the World Wildlife Fund's International Arctic Programme, 'Human-induced climate change has brought humanity to a position where we shouldn't exclude thinking thoroughly about this topic and its possibilities.'